As will be apparent inter alia from the aforementioned copending applications and the related patents, the thermal printing process generally makes use of a ribbon having a support generally in the form of a carrier foil and a melt color layer applied to the transfer side of this foil.
The ribbon is displaced between a thermal printing head and a substrate adapted to receive the print and the printing head is activated to apply a head symbol or character, e.g. an alphanumeric symbol to the reverse of the ribbon and thereby melt over a corresponding pattern the melt layer on the obverse side and transfer the melted portion to the substrate. The substrate can be a sheet of paper which can be passed between the ribbon and a platen or other support.
The print head may utilize pins and the printing apparatus can be a dot matrix printer, or the symbol can be provided on a carrier such as a wheel, band or thimble.
Printers of this type can be integrated with keyboards or numeric pads, e.g. in office machines such as typewriters and calculators. They may also serve as printers connectable to computer or word processor terminals and the like.
By and large, the carrier foil is a plastic foil or fabric, similar to those used for carbon ribbons, while the melt color or transfer layer contains a wax and/or a waxlike substance, a dyestuff or other color agent and a thermoplastic binder together with optional additives.
Thermoprint ribbons of the aforedescribed type have, of course, long been known. Usually they make use of a foil-like carrier support which can be composed, for example, of paper or a synthetic resin, and a layer forming the transfer layer which constitutes the so-called melt color. The reference to a "melt color" is, naturally, merely shorthand for the statement that a fusible color layer can be melted onto a substrate to leave a portion of the fusible color layer on the substrate in the pattern of the head symbol applied to the back or reverse side of the ribbon.
It has already been mentioned that the fusible color layer will generally comprise a meltable wax-bound or synthetic resin bound dyestuff or carbon black layer.
As noted, the fusible color layer is melted by the hot printing head and a melted portion transferred to a substrate which can be a paper or foil.
The ribbons used heretofore in this manner have been termed thermotransfer or TCR ribbons, the latter acronym being short for "thermal carbon ribbon".
Thermal printers in which a hot symbol is printed in the manner described, have been the subject of German Printed Applications 2,062,494 and 2,406,613 as well as German Open Application 32 24 445.
In the operation of such printers, the printing head should generally be capable of developing a temperature which, at its maximum, will be about 400.degree. C. The uncoated backside or reverse of the thermocolor ribbon, generally the reverse of the foil carrier, will come into direct contact with the printing head during the printing operation and, of course, the hot symbol.
At the instant of printing the relative speed between the thermocarbon and the paper or foil to receive the imprint is usually zero. Upon contact of the printing symbol with the ribbon, melting of the fusible color layer and contact of the melted portion thereof with the substrate paper or foil, a pattern of the color layer corresponding to the symbol will be transferred to the substrate. Upon detachment of the ribbon from the substrate, the previously melted material in the shape of the symbol will remain adherent to the substrate and will congeal.
Apart from the thermocolor ribbons described above with simple foil-like carriers, there are also thermocolor ribbons in which the hot symbol is not formed by a thermal printing head but by resistance heating of specially formed foil-like carriers. The melt color which is also the functional layer, during this printing process, also is transferred to a substrate in the desired pattern. In the trade, such ribbons are referred to as electrothermal ribbons and the process as an electrothermal transfer process (see U.S. Pat. No. 4,309,117)
Multiuse thermal transfer ribbons, i.e. thermal transfer ribbons capable of multistrike capacity, are described for example in EP-A-0 063 000. The fusible color layer of the ribbon here is in the form of a particulate material which is insoluble in the solvent of the coating liquid and does not melt at temperatures below 100.degree. C. The coating liquid also contains a further particulate material with a melting point between 40.degree. and 100.degree. C. The particulate material which does not melt at temperatures below 100.degree. C. is preferably a metal oxide, a metal, an organic resin or carbon black. Because of this special relationship of the particulate materials, the fusible color layer is a solid mixture with a heterogeneous structure designed so that at each printing strike, only a small amount of the fusible colored material is consumed by transfer to the substrate.
The conventional methods of making the abovementioned thermocolor ribbons possess, inter alia, the significant drawback that they make use of solvents which are released into the environment and are considered contaminants of the workplace or of the environment.
German Patent Document DE-OS 36 26 467 describes a process for thermal transfer in which the thermocolor ribbon does not require the use of an environment contaminating solvent.
However, the ribbon here is only a single strike ribbon and is not designed for multistrike or multiuse capability.